Lightning protection of structures and electronic equipment: a case study.
Lightning-related damages to structures and electronic equipment can be mitigated by following lightning protection techniques recommended in recognized national and international standards. However, field engineers in many parts of the world doubts the effectiveness of these design practices and are reluctant to use them. The paper uses a case study to demonstrate the effectiveness of some standard methods for lightning protection system for structure and equipment. The case study was undertaken at the premises of a gold mining company located in an area of high lightning activity. Frequent damages to the company's electronic and telecommunication equipment and structures became a major concern. The paper presents and discusses the techniques applied to mitigate the problem and reports the results achieved which demonstrate the effectiveness of the techniques.
Keywords: Lightning Protection System, Grounding System, Rolling Sphere Method, Critical Resistance of Electrode.
Lightning is a phenomenon that has often caused severe damage to life and property. Direct lightning strike on structure results in an impulse current ranging from around 3kA to 200kA. Long duration impulse current can cause fire whereas short duration high current peak tend to tear or bend metal parts . The best protection against direct strike is still the conventional lightning protection system which serves to provide lightning attachment points and paths for the lightning current to be safely dissipated into the ground. The system basically consists of air terminals deployed at appropriate points on the structure to intercept the lightning current, ground or earth electrodes to dissipate the lightning current into the earth and down conductors to carry the lightning current from the air terminals to the ground electrodes. The design and positioning of the air terminals are achieved using the Rolling Sphere Method (RSM). This method is widely recognized in national and international standards and its effectiveness has been verified [2, 3].
Damages to electronic equipment can also result from indirect lightning strike. The indirect strike induces transient over voltages and currents which reach the electronic systems via power supply, data, communication and signal lines. The lightning-induced over voltages can arise from direct lightning strike on overhead lines, rise in ground potential due to discharge of lightning current into the ground or intense electromagnetic fields radiated by lightning flashes. Most of the electronic equipment damage is caused by ground potential rise. Electrical damage from ground potential rise (GPR) has an estimated cost in the millions of dollars a year, but few field engineers or service managers in the industry are even aware of the phenomenon . The GPR may not destroy equipment immediately but may weaken individual components that fail later.
A series of field-proven national standards provides methods for protecting structures and equipment from direct lightning and GPR effects. These documents have existed for years, but most field engineers and technicians doubt their effectiveness and are reluctant to use them. For example, many national standards and research articles recommend the interconnection of all earthing systems such as lightning protection, power and communication, of a given installation to have a single earth. In other part of the world, it effectiveness is being doubted .
In the case study, certain number of the above lightning techniques has been employed to check physical damage to structures and failure of electronic systems at the premises of a mining company located in a lightning prone area having isokeraunic level of 160 . The paper presents the techniques as applied and the results achieved.
Application of The Rolling Sphere Method
The RSM concept is based on electrogeometric model EGM. This model is based on striking distance concept. From the physics of lightning strike, a streamer emitted by an earth-based object cannot propagate to a descending leader until the electric fields between the object and the streamer are sufficiently high [2, 7]. The fields are proportional to the amount of charge. Also, the peak current of a lightning strike is proportional to the leader charge . Thus, the striking distance is related to the lightning current. The striking distance is greater for larger current discharges [2, 7]. Use of the EGM and the empirically-derived striking distance provides a method for calculating the placement of air terminals to collect lightning current above a desired threshold, the RSM being a simple method for applying the EGM. The use of RSM for the design and position of air terminals involves rolling an imaginary sphere of a radius equal to an assumed striking distance over the air termination network. The sphere rolls up and over air terminals and other grounded metal objects intended for direct lightning protection. A piece of equipment is protected from a direct stroke if it remains below a curved surface of the sphere by virtue of the sphere's being elevated by the air terminals. Equipment that touches the sphere or penetrates its surface is not protected. The RSM generally assumes the striking distance to be the same for all land-based objects irrespective of their geometry and for the earth itself .
Application of the RSM to air terminal in the form of Franklin rods could be simplified by deriving an analytical formula for the zone of protection for a single mast defined by the RSM. Referring to Fig. 1, the distance R known as lateral distance or attractive radius for the mast can be obtained from the equation
[d.sub.s.sup.2] = [R.sup.2] + [([d.sub.s] - H).sup.2] (1a)
where [d.sub.s] is the striking distance and H is the height of the mast as
[R.sup.2] = 2[d.sub.s] H - [H.sup.2] (lb)
R = [square root of H(2[d.sub.s] - H) (1c)
Also for a distance r away from the mast, the height h of an object that touches the curve surface of the rolling sphere can be obtained from
[d.sub.s.sup.2] = [(R - r).sup.2] + [([d.sub.s] - h).sup.2] (2a)
[(R - r).sup.2] = [2d.sub.s] h - [h.sup.2] (2b)
[FIGURE 1 OMITTED]
The radius r is termed the radius of protection for an object of height h. Dividing Eq. (1b) by Eq. (2b) and simplifying gives Eq. (3) which can be used to calculate the radius of protection for an object of a given height h under a lightning rod or mast of height H.
r = R[[1 - [square root of 2[d.sub.s] h - [h.sup.2]/2[d.sub.s]H - [H.sup.2]] (3)
For example, if a lightning rod is set at 12 metres above local earth, then for a striking distance [d.sub.s] of 33m, the attractive radius R = 25m(from Eq. (1c)) and for an object of h = 1.5 m, its radius of protection r = 15.3 m (from Eq. (3)).
Fig. 3.3 shows a graphical representation of Eq. (3) for two cases: H = 30 with a striking distance of 33 and H = 12 with a striking distance of 60.
[FIGURE 2 OMITTED]
For a design of lightning protection system, International Electrotechnical Commission (IEC 61024-1) presents four protection levels for the protection of structures (see Table 1). The level of protection chosen gives the minimum peak current expected to be intercepted by the lightning protection system and this is turn used to derive the rolling sphere radius.
Electronic Equipment Protection
Surge protective devices [SPDs] properly applied to electric circuit can protect electronics from most of the secondary effects of lightning. More than 90% of SPDs are based on Metal Oxide Varister MOV component connected across the terminals to be protected . The MOV is non linear device which allows large amounts of surge current to flow through it whilst maintaining a relatively small voltage across it. SPDs work by detecting over voltages, and shorting them out. Properly chosen and installed MOV demonstrates an excellent ability to clamp over voltages to a safe level absorbing excess of the transient energy associated with lightning. However, no surge protective device ever made will protect electronic equipment from a ground potential rise [4, 9]. During a ground potential rise, the surge protective devices, merely offer an additional current path off the affected site to remote ground. They actually provide a path for current to flow in the reverse direction . GPR effects can be mitigated by diligent application of the following techniques:
Divide and Control
The control of dissipating lightning-strike energy requires division . This is an absolute must for success because of the magnitude of the current and the resulting surge impedance of any single dissipation path. Electrode system should be well spread in order to have fast and less concentrated dispersion of charge brought by the lightning current, into the mass of earth. According to Gomez , crowded configuration of earth electrodes or a single earth electrode may not facilitate such charge dispersion. A radial electrodes connected to a ground ring bonded together is one sure way of dividing the lightning current . For example, ten radial electrode bonded to a ground ring will divide lightning current up into 10 smaller segments. This will help ensure that the lightning will more likely follow the designated paths for dissipation into the earth, and lower the resulting GPR. According to More et al , the optimum length of such radials is approximately 8 metres, with interconnecting 1.4m ground rods . Longer-length radials will offer little dissipation improvement because the lightning-strike energy will not remain on the radials for much over 24 m . The ground potential rise and a potential profile over soil during striking event are much more reduced for this configuration in comparison with others .
Reducing Ground Resistance Value
GPR is directly proportional to earth electrode resistance and the current flowing through it . Hence, any attempt to reduce the earth electrode resistance is likely to lower the GPR value and consequently reduce its effects. There are a number of methods which can be used to reduce earth resistance of earth electrodes on sites where soil resistivity is high, replacement of the soil within its effective resistance area with more conductive soil being one of them. Replacing the soil in the entire effective resistance area is in general not practical. The effectiveness of replacing soil in the critical resistance area only is one technique that can be applied .
Lightning potential equalization is considered indispensable in electronic equipment protection. This is believed to be of greater importance than the ground resistance of earthing system . Single point grounding is an absolute must to prevent GPR and from damaging equipment . This is achieved by establishing what is called a common reference bar. Every earth conductor that enters or leaves a 'service entrance point' is bonded to the reference bar. This means earth of power, phone, coaxial and control cabling are all bonded together at the reference bar. The reference bar is then grounded through a lowest inductance connection. Current cannot flow between two points of equal potential, even if the equipment rises to several thousand of volts during strike, no voltage differentials will exist that can drive damaging currents through them.
The case study was undertaken at the administration area of the Bibiani Mines, a mining company located in the western region of Ghana. The administration block houses over 60% of the company's electronic equipment and this equipment must work during the most difficult times including bad weather conditions. The company had been plagued with high incidence of electronic equipment failures including damage to a satellite dish by direct lightning strikes. Direct strike on the satellite dish had occurred at least once in every two years resulting in physical damage to the dish. In this study, design of air termination network with the RSM was undertaken. High earth resistances were lowered by backfilling critical resistance areas of earth electrodes. GPR effects were reduced with vertical and horizontal ground electrode system and finally all the grounding systems and metallic structures were bonded together to eliminate incidence of potential differential. Results achieved demonstrated the effectiveness of these techniques.
Layout of the administration area
Located in the administration area is administration building; an outdoor power station, a communication tower and a satellite dish. The area layout is shown in Fig 3
[FIGURE 3 OMITTED]
A 3-phase, 200kVA, 33/0.4 kV, delta-star transformer at the power station supplies power to the building through an Automatic Voltage Switcher (AVS) installed at the service entrance of the building. The AVS feeds a 10kVA interruptible power supply (UPS) and this in turn supplies power to computers, office equipment and private automatic business exchange (PABX) (see Fig. 4). Lines from the PABX are protected by data line protector (DLP). A coaxial cable from the satellite dish goes to the main internet server in the administration building. Its sheath, before the study, had been grounded at the base of the satellite dish where earth resistance reading was 14 ohms. Another coaxial cable leaves the communication tower for the PABX. A lightning protection system was provided for the administration building and a separate protection system for the communication tower was supposed to take care of the satellite dish. The lightning protection for the administration building consisted of two Franklin rods, separately grounded via two down conductors. The tower had one Franklin rod installed with the tower serving as down conductor and its footings as grounded electrodes. On the 33-kV side of the transformer is installed a lightning arrestor of IOkA, 8/20[micro]s rating
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
The case study was carried out in mid 2004. Before then, the lightning protection measures in place had not been effective in checking lightning-related damages to structures and equipment. Financial statistics available indicate that the existing protection system resulted in about 10% reductions in the annual maintenance cost . The ineffectiveness of the existing protection system could be attributed to improper design of air terminals and grounding systems and inadequate protection against ground potential rise.
The following steps were taken to improve the effectiveness of the total lightning protection at the area.
Step I: The satellite dish and the tower, 2m and 30m tall respectively, are spaced at a distance of 30m. From Eq. (3), the dish could be considered protected for an assumed striking distance of 60m which corresponds to a protection level IV (see Table 1). The dish was relocated 15m from the tower to give it a protection level of II, the corresponding striking distance being 30m.
Step 2: The lightning protection of the administration building was improved to level IV by erecting four 12-m masts at corners of the building. The ridge of the building roof is about 6m above the ground.
Step 3: Earth electrode resistances at various sites were improved by installing additional earth rod electrodes and backfilling the critical resistance area of the electrodes with low resistivity soil . The old and new earth electrode resistance values at the various sites are given in Table 2.
In addition, the grounding systems at the administration building and at the tower base were configured as shown in Figs. 5 and 6 to take advantage of the horizontal nature of lightning discharge into the ground and to also improve on a ground potential profile and reduce the GPR effects . A 75 [mm.sup.2] bare conductor was used to ring the ground electrodes and radials added to the administration grounding systems, again to reduce the GPR effects.
Step 4: At the transformer station, the arrestor earth, the transformer tank and the transformer neutral earth were bonded together resulting in overall earth resistance of 5 ohms and at the service entrance point, the sheath of the coaxial cables and all earth terminals of equipment were connected to an earth reference bar which was in turn bonded to the lightning grounding system at the building.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Results and Discussion
Fig. 7 shows the insulation on a 75 [mm.sup.2] copper grounding conductor burnt by lightning current in one incident of direct lightning strike on the tower. This was after the redesign of the lightning protection system. This lightning current was intercepted and discharged into the ground without any of the electronic equipment suffering damage. There has also not been any direct strike on the satellite dish.
For a critical evaluation of the protection performance, data on electronic equipment failure rate before and after the redesign were taken from the company's maintenance and operations report and analyzed . The results of the analysis are reported in Table 3 and Fig. 8. The failure rate of equipment type is defined as the ratio of the number of times the type has suffered damage to the total number of the type in place.
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
To quantify the degree of overall protection performance offered by the protection system, average equipment failure rate for 2002 and 2003 were compared with average failure rate for 2004 and 2005. The results are reported in Table 4 below. Fig. 9 and 10 below also throws more light on table above for it to be absorbed easily.
Protection performance was calculated using the following equation:
Protection performance = [avf.sub.1] - [avf.sub.2]/[avf.sub.1] X 100%
[avf.sub.1] is the average failure rate between 2002 and 2003
[avf.sub.2] is the average failure rate between 2004 and 2005
As can be noticed in Table 4, there is an overall average protection performance level of 75%. Financially, a total savings of $80,000 was recorded in the annual equipment maintenance cost over the period of 2004 and 2005 . As compared with the previous performance of 10% , the 75% improvement was significant and explains why diligent application of recognized lightning protection standards still remains the best.
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
Technical measures implemented in an attempt to check the damages of electronic equipment and structures of a company have been presented. Results achieved in terms of the control of equipment failure rate and the elimination of direct hits on structures to date has proved that the measures are effective. It is observed that there is still room for improvement and this may be achieved by appropriate installation of suitable surge protection devices.
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P.Y. Okyere and George Eduful *
Kwame Nkrumah University of Science and Technology
Private Mail Bag, Kumasi-Ghana
* Corresponding Author E-mail: firstname.lastname@example.org
Table 1: Lightning Protection Level, Efficiency Protection Rolling Sphere Protection Level Effectiveness Radius I 98% 20m II 95% 30m III 90% 45m IV 80% 60m Table 2: Results of Earth Resistance Improvement Earth Resistance in ohms Site Previous Present Tower 13 5 Building 31 1.7 arrestor 87 5 neutral 110 5 Table 3: Results of Equipment Failure analysis Failure Rate Types of equipment 2002 2003 2004 2005 Computers 54% 47% 29% 18.50% Printers 39% 20% 10% 10% Photocopier 66% 67% 0% 0% Fax machine 20% 50% 0% 0% PABX 120% 88% 50% 37.50% Internet link device 100% 67% 25% 0% servers 200% 0% 0% 0% Phones 55% 41% 50% 21% Table 4: Overall Protection Performance Equipment Average Average Protection failure failure performance (%) rate for rate for 2002 and 2004 and 2003(%) 2005 (%) Computers 50.5 23.75 58 Printers 29.5 10 66 Photocopier 66.5 0 100 Fax machine 35 0 100 PABX 104 43.75 60 Phones 48 23 52 Internet link Device 83.3 25 70 Computer server 100 0 100 Average protection performance 75%
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|Author:||Okyere, P.Y.; Eduful, George|
|Publication:||International Journal of Applied Engineering Research|
|Date:||Jan 1, 2007|
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